In the field of electro-static discharge (ESD) protection designs, a silicon-controlled rectifier (SCR) has been widely recognized for its characteristic of a high ESD discharge capability, but there are two serious defects in this type of device limiting its applications: the first defect is a high triggering voltage for a snapback effect due to the fact that its triggering voltage is mainly limited by a high reverse breakdown voltage of an N-Well to a P-Well; and the second defect is a very low holding voltage for the snapback effect, which easily leads to a latch-up effect.
With regard to the defect of a high triggering voltage, various schemes have been proposed in the industry to reduce the triggering voltage for the snapback effect, such as the silicon-controlled rectifier structures shown in FIGS. 1 and 2. In the silicon-controlled rectifier shown in FIG. 1, between an N-Well and a P-Well, an N-type heavily-doped region spans the N-Well and the P-Well is inserted, so as to achieve the purpose of reducing a reverse breakdown voltage of the N-Well to the P-Well. In one embodiment, the silicon-controlled rectifier as shown in FIG. 1 comprises a P-type substrate 100, an N-type doped N-Well 210, a P-type doped P-Well 220, and an N-type heavily-doped region 412 and a P-type heavily-doped region 422 in the N-Well 210, an N-type heavily-doped region 414 and a P-type heavily-doped region 424 in the P-Well 220, and an N-type heavily-doped region 410 spans the N-Well 210 and the P-Well 220 at a position where the N-Well 210 abuts the P-Well 220. The N-type heavily-doped region 412, the P-type heavily-doped region 422, the N-type heavily-doped region 410, the N-type heavily-doped region 414 and the P-type heavily-doped region 424 are spaced apart by shallow trench isolations (STI) 300. The P-type heavily-doped region 422, the N-Well 210 and the substrate 100 form an equivalent PNP bipolar structure, the N-type heavily-doped region 412 and the N-Well 210 form a diffusion resistance equivalently connected to a base of the above-mentioned PNP bipolar, the P-type heavily-doped region 422 forms an emitter of the above-mentioned PNP bipolar, the substrate 100 is a collector of the above-mentioned PNP bipolar, and the N-Well 210 is the base of the above-mentioned PNP bipolar. The N-Well 210, the substrate 100/the P-Well 220, and the N-type heavily-doped region 414 form an equivalent NPN bipolar structure, the N-Well 210 forms a collector of the NPN bipolar, the N-type heavily-doped region 414 forms an emitter of the NPN bipolar, and the substrate 100/the P-Well 220 forms a base of the NPN bipolar. Moreover, the N-type heavily-doped region 412 and the P-type heavily-doped region 422 are connected to an anode of the silicon-controlled rectifier, and the N-type heavily-doped region 414 and the P-type heavily-doped region 424 are connected to a cathode of the silicon-controlled rectifier.
The silicon-controlled rectifier shown in FIG. 2 is formed on the basis of the silicon-controlled rectifier in FIG. 1, by removing a shallow trench isolation 300 between the N-type heavily-doped region 410 and the N-type heavily-doped region 414, and forming an N-type gate 430 on the surface of the substrate corresponding to a position where the shallow trench isolation 300 is removed and connecting the N-type gate to the cathode of the silicon-controlled rectifier, thus forming an N-type gated diode together with the P-Well 220. In the silicon-controlled rectifier as shown in FIG. 2, the N-type gated diode is introduced to further reduce the reverse breakdown voltage of the N-Well to the P-Well, but even so, the triggering voltage of the silicon-controlled rectifier shown in FIG. 2 is still high, for which, due to the limit by the existing process parameters, the adjustment freedom is not large and cannot meet the actual requirements.
With regard to the defect of a very low holding voltage for the snapback effect of the silicon-controlled rectifier, an ESD protection structure of the silicon-controlled rectifier as shown in FIG. 3 is further proposed in industry. Compared with the silicon-controlled rectifier as shown in FIG. 2, in the silicon-controlled rectifier shown in FIG. 3, the N-type heavily-doped region 410 spans the N-Well and the P-Well connects with the anode of the silicon-controlled rectifier directly, which is introduced to reduce the triggering voltage for the snapback effect of the silicon-controlled rectifier, and the P-type heavily-doped region 422 are directly connected to the anode of the silicon-controlled rectifier together, while the N-type heavily-doped region 412 connected to the anode of the device in FIG. 2 is removed. Since the N-type heavily-doped region 410, which has a high doping concentration and is provided between the N-Well 210 and the P-Well 220 in the silicon-controlled rectifier as shown in FIG. 3, is directly connected to the anode, the triggering voltage for the snapback effect of the silicon-controlled rectifier is directly determined by a breakdown voltage between the N-type heavily-doped region 410 and the P-Well 220, and is greatly reduced. Moreover, since the N-type heavily-doped region 410 is directly connected to the anode, an applied positive voltage can reduce the probability of holes being injected and migrated from the P-type heavily-doped region 422 to an N-well 210/P-well 220 interface, so that in the ESD protection structure of the silicon-controlled rectifier, the current gain of the parasitic PNP bipolar composed of the P-type heavily-doped region 422, the N-Well 210 and the P-Well 220 is greatly reduced, and thus the holding voltage for the snapback effect of the silicon-controlled rectifier as shown in FIG. 3 is increased accordingly.
FIG. 4 shows a snapback effect curve and a snapback effect electric leakage diagram of the silicon-controlled rectifier as shown in FIG. 3 obtained in a certain process platform, wherein a curve with diamond legends is the snapback effect curve of the silicon-controlled rectifier as shown in FIG. 3, and a curve with square legends is the snapback effect electric leakage curve of the silicon-controlled rectifier as shown in FIG. 3. It can be seen from the snapback effect curve of FIG. 4 that the triggering voltage of the silicon-controlled rectifier as shown in FIG. 3 is 8.4 V, which is less than a transient breakdown voltage of 11.6 V of a gate oxide layer of a 2.5V/3.3V peripheral interface circuit device in the process platform, and from the viewpoint of the triggering voltage, the silicon-controlled rectifier as shown in FIG. 3 is already applicable to the platform. However, the holding voltage of the silicon-controlled rectifier as shown in FIG. 3 is 3.2 V; although the voltage is greater than the maximum operating voltage (Vddmax=2.75V) of the 2.5V peripheral interface circuit in the process platform, it is still less than the maximum operating voltage (Vddmax=3.65V) of the 3.3V peripheral interface circuit in the process platform.
This indicates that although the silicon-controlled rectifier as shown in FIG. 3 is already completely applicable to an ESD protection circuit design of the 2.5V peripheral interface circuit in the process platform, for an ESD protection circuit design of the 3.3V peripheral interface circuit in the process platform, its holding voltage needs to be further increased above 4V.
Therefore, there is an urgent need for a new silicon-controlled rectifier, which can further reduce the triggering voltage for the snapback effect, and further increase the holding voltage.